Network analyzers measure frequency characteristics of circuit networks or circuit components by sweeping a measurement signal over a wide range. In order to measure frequency characteristics, a vector voltmeter which measures the amplitude and phase of the measurement signal, must be highly accurate over such a wide range. However, since it is difficult to make vector voltmeters with high accuracies over the entire range of measured frequencies, the measurement signal is generally converted to a constant frequency by a frequency conversion means (i.e., through a heterodyne method) and is then measured by the vector voltmeter.
FIG. 5 shows the most basic structure of a network analyzer which uses the heterodyne method. A signal source 10 comprises a measurement signal source 19 and a local signal source 20. Measurement signal source 19 generates a measurement signal and applies it to an object of measurement 15. Local signal source 20 generates a signal with a frequency that is separated from the measurement frequency by an intermediate frequency. The response signal, resulting from application of a signal to measurement object 15, and the signal from local signal source 20, are mixed by a mixer 16, converted to an intermediate frequency and output to a filter 17. Filter 17 enables passage of signal frequency components at the intermediate frequency. Vector voltmeter 18 measures the amplitude and phase of the measurement signal that has been converted to the intermediate frequency. Since vector voltmeter 18 performs measurements at the constant intermediate frequency, highly accurate measurements are achieved.
In applying the heterodyne method to a network analyzer, it is important to generate a local frequency that is accurately displaced with respect to the sweep measurement frequency by an amount of the intermediate frequency. FIG. 6 shows a prior art example of a method of generating the measurement frequency and the local frequency. FIG. 6 shows only signal source 10 part of FIG. 5.
FIG. 6 illustrates a first fixed-frequency signal source 21, a sweepable variable-frequency signal source 22, a second fixed-frequency signal source 23, a first mixer 24, a second mixer 25, a first filter 26, and a second filter 27. The signals of first fixed-frequency signal source 21, with a frequency fc, and variable-frequency signal source 22, with a frequency fd, are mixed by first mixer 24, and signals with the frequencies fd.+-.fc are output. The fd+fc signal is removed by first filter 26, and the signal with frequency fd-fc is output as the measurement signal. Therefore, the frequency of the measurement signal varies according to changes in the frequency fd of variable-frequency signal source 22.
In the same manner, a frequency fd of variable-frequency signal source 22 and a frequency fe of second fixed-frequency signal source 23 are mixed by second mixer 25, and a frequency fd.+-.fe is output. Second filter 27 removes the component with the frequency fd+fe, in the same manner as the first filter, and the signal with the frequency fd-fe is output as the local signal. The frequency of the local signal also varies according to the changes in the frequency fd of variable-frequency signal source 22.
The difference in the frequencies of the output signal of first filter 26 and second filter 27 is fc-fe; this is a constant frequency, without relation to the frequency of the measurement signal. Therefore, if the measurement signal and the local signal are mixed by mixer 16 of FIG. 5, an intermediate frequency signal with a constant frequency is output.
The prior art shown in FIG. 6 is an ingenious method for generating two sweep frequencies with a constant frequency difference, but it has the following drawbacks. First, a plurality of high-frequency signal sources, composed of complex circuits, are required. Further, since the desired frequencies are generated by mixing frequencies using a plurality of mixers, many frequency components other than the desired frequencies are generated. As a result, a complex means of removing errors due to these frequency components is required. Because of this complex circuit assembly, the prior art has the problem of high cost.
Accordingly, it is an object of this invention to provide a simple signal generation method, in order to reduce the cost of a network analyzer.